2001 van 5.2,,new oil & filter, mechanical oil gage installed,,, oil 40 psi ,, exit freeway , now doing 35 mph, oil drops to 20 psi and stays there. Increased speed to 50, oil still at 18-20 psi.,, pulled over and turned off engine, restarted ,, oil psi back to normal. Third time its done this in couple weeks.I am thinking oil pump relief staying open , not closing, to increase pressure

 

Either that, or the pickup is getting clogged.... or something floating in the pan.

 

When I first got my 3.9 I did have the oil pressure gauge going to zero a too, but only twice.
After the second time, I checked It out thoroughly and couldn't really find anything wrong.
So I changed the oil and went with Mobile1. It only did it once about 4 days later. So I changed the oil again with Mobil 1.
Never did it again after that. I think there was some type of sludge that was clogging the oil send unit...Im not sure, but I didn't have an issue again. Anyway 6 or so months later, when I overhauled the 3.9 and I had the oil pump replaced. I never had an issue again.

 

UPDATE,, removed oil pan, intake screen had black hard carbon restricting flow,, all good now

 

Cool!

I believe in these Magnum motors, conventional should be replaced every 3k and syn every 5-7k...
Long times on conventional would do that exact thing...glad you got it sorted out.